Editor Charles H. Smith's Note: A note printed in the 16 February 1871
number of Nature. Original pagination indicated within double brackets.
To link directly to this page, connect with: http://people.wku.edu/charles.smith/wallace/S184.htm

[[p. 309]] Mr. Croll's
papers on Ocean Currents are a powerful application of the modern theory
of heat and force, to show the fallacy of Captain Maury's explanation
of the causes of oceanic circulation. They also discuss other matters
of great interest, but as the concluding part is not yet published, we
shall say no more about them at present, but that they well deserve careful
study.

The other paper is a criticism of the Rev. Canon
Moseley's supposed proof that glaciers do not descend by the force of
gravity, and of the arguments of Messrs. Ball and Matthews on the other
side. It will be remembered, that Canon Moseley determined by experiment
the "shearing" force of ice, that is, the force required to fracture it
by parallel pressure. A plug of ice of known cross-section is fitted into
a hole through two smooth boards, and the force required to break the
ice by sliding the boards over each other is the "shearing" force. Increasing
this in proportion to the dimensions of a glacier, or of any large portion
of one, it was calculated that the force required to cause the different
parts of a glacier to slide over each other (as they must do in descending
a valley of constantly varying form and size) was at least thirty times
greater than the force of gravity on a slope such as glaciers easily descend.
Canon Moseley came to the conclusion that expansion and contraction of
the ice by heat and cold was the moving power; and the fact that the glaciers
move slower by night than by day, and in winter than in summer, was supposed
to prove conclusively that heat is the cause of motion.

Mr. Croll believes that Canon Moseley has demonstrated
that gravity alone does not cause glaciers to descend, but he completely
demolishes the theory of contraction and expansion. He admits that heat
aids the motion, but maintains that it does so by acting on the molecules
of [[p. 310]] the ice, which it loosens momentarily
from their mutual cohesion, and allows to be re-arranged under the influence
of gravity. Heat, he says, is the condition, gravity the cause
of the motion which takes place, molecule by molecule rather than in masses.
It seems very doubtful, however, if this theory is more tenable than the
one it is intended to supersede. If heat entering the glacier loosens
the molecules in its passage and enables them to move insensibly into
new positions, it is difficult to understand what causes the numerous
longitudinal and transverse fissures of a glacier, the production of which
is often attended by loud reports, and which indicate movements of masses,
not of molecules. And how could molecular motion lead to that heavy grinding
of the ice over its bed, which scores and wears down the hardest rocks,
and whitens great rivers with the finely triturated mud?

None of the opponents of Canon Moseley have
noticed what seems to the present writer to be a radical fallacy in his
argument about "shearing force." He assumes that, whatever the bulk or
weight of the glacier, or of any portion of it to which the formula of
the shearing force may be applied, the whole mass shears at once by the
action of gravity on the same mass, and does not recognise the possibility
of one portion of a glacier acting by its weight to shear another and
much smaller portion. But this must inevitably occur; for, owing to the
excessive irregularity of the bed in which every glacier moves, the mass
must be every where in varying states of tension and compression, and
must contain at each instant certain lines and planes of least resistance,
the extent of which lines and surfaces may be very small compared with
the dimensions of the glacier itself. At any moment, therefore, the whole
descending weight of a portion of the glacier containing perhaps thousands
of cubic yards of ice, may act so as to cause the shearing of a few superficial
feet where the tension is greatest. This being effected, a partial equilibrium
is produced there; but the points or surfaces of greatest tension are
shifted, and another small shear or fracture occurs; and by this process
and the continued regelation of fractured surfaces brought into contact,
it may easily be seen that the glacier as a whole would be gradually moulded
to its bed, which it would descend as surely as if it were a viscous mass.
Another source of motion not taken into account either by Canon Moseley
or Mr. Croll is the irregular melting away of the under surface of the
glacier by terrestrial heat, which would often form unsupported hollows
till a fracture occurred, and every such fracture must result in a downward
motion of a portion of the glacier. The observed difference of the rate
of motion between winter and summer, day and night, is more probably due
to the different quantities of water which descend the crevasses into
the bed of the glacier at those periods, than to any direct action of
the heat. It is well known that in the higher portions of a glacier the
supply of water from melting snow diminishes during the night, as it does
in a still greater degree during the winter; and the large quantity of
water that flows beneath every glacier in the summer must greatly assist
its motion, both by melting away its lower surface, and by, to some extent,
buoying it up.

Mr. Matthews's important experiment of the bar
of ice which gradually curved by its own weight, should be tried again
in an atmosphere kept at the freezing point. This would settle the question
whether heat is an essential condition for the curvature or motion of
ice by gravitation; but so far as the facts lead us at present, the arguments
of Canon Moseley and Mr. Croll by no means prove that glaciers
do not descend by the force of gravity alone.

[The publication of this article has been delayed. It was in our hands
before the appearance of Mr. Ball's paper in the Philosophical Magazine
for February, where a view almost identical with Mr. Wallace's is ably
advocated.--Ed.]

Note Appearing in the Original Work

1"On
Ocean Currents." By James Croll, of the Geological Survey of Scotland
(3 parts). "On the Cause of the Motion of Glaciers." By the same author.
(Extracted from the Philosophical Magazine of 1870.)

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Comment by Doug Benn, University of St. Andrews, Scotland
(pers. commun. 12/00):

The debate on the causes of glacier motion lasted for many years
during the 19th Century. Major sticking points were (1) the fact that
ice appears to be a rigid, brittle material, yet the large-scale motion
of glaciers requires that the ice flows as a continuous mass; and (2)
a mass under a constant force (gravity) should be expected to accelerate
downhill (as all falling objects do) whereas glaciers exhibit periodic,
often annual, fluctuations in speed. Wallace's note covers a range of
issues connected with these problems. He correctly dismisses some of the
fallacies current at the time, giving reasons why ice motion cannot be
due to freeze-thaw action or "molecular adjustment." His own preferred
mechanism, of localised fracture and regelation (healing of fractures
by refreezing) is closer to the mark, but still not complete. In fact,
the correct mechanisms of ice flow--involving visco-plastic deformation
in response to gravity--had been recognised by James David Forbes some
years previously, although serious problems remained with ice flow theory
that were not really addressed until the 1950s.

Where Wallace does hit true, however, is his recognition of basal
sliding as the main mechanism by which Alpine glaciers move. His comments
that meltwater reaching the bed will encourage motion by "melting away
its lower surface, and . . . to some extent, buoying it up" are exactly
correct. This is indeed the reason why Alpine glaciers flow more rapidly
during the day and in summer, when meltwater fluxes are highest. Confirmation
of this fact had to await the development of drilling equipment, allowing
the real-time measurement of subglacial water pressures simultaneously
with accurate velocity measurement.

Wallace was not the first scientist to emphasise basal sliding (de Saussure
has this honour), but he certainly had a more lucid view of the problem
than many of his contemporaries. Forbes, an otherwise perceptive field worker
and theorist, had dismissed sliding as unimportant.